Medical and dental implants are widely used today. Typical examples would be the implants used for joint replacements, fractures fixation, and bone reconstruction. Joint replacement implants typically comprise two parts: a metal or ceramic part formed with an articulating surface designed to be received in and rub against a complementary load-bearing plastic surface of either an all-plastic part or a metal part with a plastic surface. In the case of metal implants, the type of metal may include titanium and/or its alloys, or cobalt-chromium and/or its alloys. Other metals useful for medical implants are also applicable. The choice of plastic for the plastic part is, for the most part, ultra-high molecular weight polyethylene (UHMWPE). Fracture fixation and bone reconstruction implants can consist of pins, screws, plates or intramedullary rods. All of these implants require fixation to the bone in order to function properly. The inability to achieve and maintain fixation results in a number of complications which include pain, implant loosening, implant failure, and compromise of function. While the examples discussed focus on medical implants, good fixation stability is an important quality for dental implants also.
The major problem inherent in all implant devices is the gradual loosening over time. This problem is especially prevalent where the implant is subjected to large functional loads and sheer stresses. Cranio-facial implants are particularly prone to this problem. For instance, the difficulty in achieving a dental prosthesis that is strongly bonded to maxillary and/or mandibular bone, and which can withstand large compressive, sheer, and tensile loads has lead to the development of a variety of attachment mechanisms. Many of these mechanisms attempt to adaptively form bone around the prosthesis, with the newly formed bone eventually bonding to the outer surface of the implant. This is especially prevalent for joint prosthesis. As a result of such surface bonding, the fixation stability of these implants is greatest in the weeks directly following implantation, while their useful lives are characterized by a slow loosening or deterioration of fixation stability. It would be desirable to design implants which could retain fixation stability for longer times, and it would be most desirable to design an implant that could actually exhibit improved fixation stability as it ages. Other orthopedic implants such as plates, screws, nails, pins, etc., are also subjected to fixation problems.
A variety of methods for promoting bone formation and attachment have been proposed. For example, U.S. Pat. No. 5,639,237 describes an endosseous dental implant having an indented surface texture for the use in tooth reconstruction. The indented surface increases the surface area for bone contact, thereby enhancing the mechanical fixation and anchoring strength of the dental implant as compared to tooth implants without indentations.
In the case of joint implant, including but not limited to hip, knee, elbow and shoulder, fixation is usually accomplished by the use of cement such as methylmethacrylate, or is achieved by a press-fit method. Both these conventional methods are usually characterized by having the greatest degree of fixation immediately after implantation, but suffer from loss of fixation in the months and years after implantation. Additionally, there is a disadvantage with the use of cements such as methylmethacrylate due to its potential toxicity.
Other approaches have attempted to strengthen the attachment of the bone at the site of the implantation. One such method is taught in U.S. Pat. No. 3,344,654, which claims that a strong bond can be achieved between existing bone and the prosthesis by coating the prosthetic device with an osteogenic protein. To enhance endochondral bone formation, U.S. Pat. No. 5,656,450 teaches compositions and methods for effecting wound healing, specifically the activation of latent growth factor through matrix vesicles. Biodegradable polymeric implants are described which may be prepared containing latent growth factor, matrix vesicles, or matrix vesicle extract. An osteogenic device capable of inducing the formation of endochondral bone when implanted in the mammalian body is also disclosed in U.S. Pat. No. 5,645,591. This device includes an osteogenic protein dispersed within a porous matrix comprising a polymer of collagen and glycosaminoglycan.
Yet another approach for improving the strength and stability of a dental implant is discussed in U.S. Pat. No. 5,383,935. According to the teachings of this patent, a prosthetic device for implantation into skeletal bone generates current flow for calcium phosphate mineral formation between the implant and the surrounding bone. The formation of calcium phosphate minerals at the implant-hone interface is described as encouraging bone attachment to the implant, thereby providing stronger fixation of the implant into the skeletal structure.
An altogether different technique for enhancing bone density at the region of the implant is described in U.S. Pat. No. 5,344,457. This reference teaches effectively transferring loading stress from a dental implant to the surrounding bone through the use of an implant having a tapered body shape. Application of a vertical force on the tapered implant produces a sheer force component in addition to the normal force component acting on the surrounding bone.
Prior to the present invention, various methods have been disclosed in the literature for the attachment of implantable devices to the musculoskeletal system. These methods can generally be classified into those involving impaction grafting, nails and screws, bone cement, and materials with surface ingrowth potential. Interest has recently been focused primarily on implants designed for fixation by tissue ingrowth into the implant's surface as representing a viable solution to the problem of late implant loosening, the most prevalent problem in joint replacement surgery using simple impaction or cementing fixation techniques. There are several types of surface ingrowth materials and methods for their fabrication that have been disclosed in the literature (Pilliar, R. M.: Surgical Prosthetic Device With Porous Metal Coating. U.S. Pat. No. 3,855,638. December, 1974; Pilliar, R. M.: Surgical Prosthetic Device Or Implant Having Pure Metal Porous Coating. U.S. Pat. No. 4,206,516. June, 1980; Smith, L. W. et al; Prosthetic Pans and Methods of Making the Same. U.S. Pat. No. 3,314,420. April 1967; Wheeler, K. R, Supp, K. R., Karagianes, M. T.; Void Metal Composite Material and Method. U.S. Pat. No. 3,852,045, Dec. 3, 1974; Frey. O.: Anchoring Surface For a Bone Implant. U.S. Pat. No. 4,272,855. June, 1981; Spector, M., et al: Prosthetic Devices Having Coatings of Selected Porous Bioengineering Thermoplastics. U.S. Pat. No. 4,164,794. August, 1979; Homsy. C: U.S. Pat. No. 3,971,670. July, 1976; Tronzo, R.: U.S. Pat. No. 3,808,606. May, 1974; Saner, B. U.S. Pat. No. 3,986,212. October 1976; and Hahn. H.: Bone Implant. U.S. Pat. No. 3,605,123. September 1974). These can generally be grouped into surface ingrowth polymers/ceramics and surface ingrowth metals. As described earlier, the porous polymers offer the advantage of allowing fabrication of a stem with lower rigidity. Their disadvantages are their generally weaker mechanical properties, their poorer biocompatibility, and their much shorter history of clinical use.
Finally, the micro-texturing of the surfaces of orthopaedic implants has been used to increase surface area for better adhesion, as well as promote bone ingrowth into the surface of device. The U.S. Patents of Wagner are exemplary for these methods; see e.g., U.S. Pat. Nos. 6,193,762; 5,922,029; 5,507,815; and 5,258,098.
Despite the plethora of existing approaches for securing an implanted structure into bone, fixation failures commonly occur. These failures are primarily due to implant loosing caused by the inability of the bone to withstand the physiological loads at the bone/implant interface. One factor in such failures is that the new bone ingrowth surrounding the implant is limited to the surface. Integration of host bone with the implant in a more seamless manner could eliminate this problem and result in better biological and physiological outcomes.
All currently available implants achieve their function by, first, establishing fixation with the host tissue. The inadequate fixation with the host tissue has been one of the major limitations of these devices. The principal components of inadequate fixation include: 1) inadequate bonding with host tissue; 2) non-optimal biomechanical properties; and 3) incompatible biologic properties. With respect to bonding of host tissue, conventional implants have been limited to bonding to the surface of the implant only, with very limited or no tissue ingrowth. Additionally, there is always an interface between the implant and the host tissue, and the interface is always biomechanically and biologically inferior to the implant and host tissue. With respect to biomechanical properties, conventional implants are bulky and stiff in order to offset fatigue, which creates a disparity between the mechanical properties of the implant and the host tissue. This disparity results in stress risers, stress shielding, and bone atrophy. Finally, with respect to the biologic properties, conventional implants either do not support or poorly support tissue biology. These conventional implants are space-occupying devices which alter local tissue biology, do not accommodate the quality of the host tissue, and do not remodel with the host tissue. Additionally, they do not allow for the application of biological factors which could enhance implant function. The result is a biological and biomechanical disparity between the implant and host tissue which culminates in loss of implant fixation.
Thus, there is a need in the medical and dental arts for improving the integrity of fixation of an implant to the host tissue. Coupled with improvements in the biological and biomechanical function, both performance and longevity of implants is possible.